Essentially all types of rubber, both natural and synthetic, and particularly rubbers formed from dienes are known to be susceptible to deterioration resulting from prolonged exposure to ozone. A great deal of effort has been expended by those engaged in the field of polymer technology to develop various antiozonants that will effectively inhibit the adverse effects of ozone on polymeric compositions. The more commercially available antiozonants for rubber and rubber goods are the several derivatives of para-phenylenediamine. These derivatives generally suffer from one or more of the following limitations: limited solubility in rubber; low but significant volatility and water solubility: reactivity toward other species such as dioxygen; staining and discoloration. One approach to decreasing the water solubility and volatility of these derivatives of para-phenylenediamine has been to increase their molecular weight.
As a class, the para-phenylenediamine derivatives are very active antiozonants but their usefulness is greatly limited by their severe staining and discoloring nature. Thus considerable effort has been expended in the search for non-staining, non-discoloring antiozonants. Many materials have been investigated for this purpose including lactams, phosphites, phosphines, thioureas, thiosemicarbazides and substituted olefins but all have significant drawbacks and limitations. The lactams which are reported to have excellent dynamic ozone protection are very scorchy. Phosphites and phosphines react readily with peroxide and sulfur curatives and so are not useful in conventionally cured goods. Of the thioureas and thiosemicarbazides the most studied are the trisubstituted derivatives especially tributyl thiourea. Chemical literature teaches that the most active materials of either class are the trisubstituted derivatives. However, like the lactams, these materials are quite scorchy and thus have found little, if any, commercial use. A substituted olefin is marked by Bayer as a non-discoloring antiozonant but it is only recommended for use in ozone-resistant rubbers such as polychloroprene. In addition to the above-mentioned materials a recent patent claims antiozonant activity for cyanoalkyl esters of dithiocarbamic acids.
This invention discloses a new class of antiozonants which are stable toward dioxygen. Their protective reactivity is specifically directed toward ozone which greatly increases their persistence in rubber. In addition, their molecular weights can be adjusted to improve volatility and water extractability characteristics. An additional benefit of these new antiozonants is that they are largely non-discoloring and non-staining. Thus, they can be used in rubber goods heretofore unable to be protected from the degradative effects of ozone, e.g. white sidewalls on tires, shoe soles, boots and rubbers, and gaskets which come in contact with painted surfaces. Other species in the class have the benefit of being less staining and less discoloring than the para-phenylenediamine derivatives commercially available.